This application is based on Japanese Patent Applications No. 2001-93519 filed on Mar. 28, 2001 and No. 2002-92332 filed on Mar. 28, 2002 the contents of which are incorporated herein by reference.
1. Field of the Invention:
The present invention relates to an airflow meter, specifically a bypass type airflow meter that has a bypass passage in which a part of air flowing through a main passage is introduced.
2. Description of Related Art:
JP-A-10-142020 (U.S. Pat. No. 5,948,975) and JP-A-11-511262 (U.S. Pat. No. 6,148,663) disclose airflow meters having bypass passages. The airflow meter disclosed in these documents has a partition member that separates the bypass passage, and a sensor element having a sensing plane. The sensor element is disposed on one surface of the partition member. The airflow meter further has a pair of tapered surfaces opposing the sensing plane of the sensor element. Therefore, the tapered surface faces the sensor element and defines a wall approaching the sensor element in a flow direction.
According to the arrangement described above, the tapered surface may direct dust and adhesive dust in the airflow toward the sensor element. The sensor element may be damaged by the dust or be polluted and covered by the adhesive dust. As a result, the sensor element may be deteriorated.
On the other hand, dust flowing in the main passage can easily reach to the sensor element, since the bypass passage in which the sensor element is disposed is parallel to the main passage and is directly opens toward upstream side of the main passage. In order to maintain a sensing function for a long period of time, it is important to protect the sensor element from dust in the airflow.
On the other hand, according to the arrangement described above, it is possible to reduce fluctuation and pulsation of airflow in a perpendicular direction of the sensing plane of the sensor element, since the tapered surface decreases a perpendicular distance between the sensor element and the tapered surface. However, airflow fluctuation in a parallel direction to the sensing plane of the sensor element still affects on the output of the sensor element.
It is an object of the present invention to provide an airflow meter which is capable of protecting the sensor element from foreign particles or dust in the airflow.
It is another object of the present invention to provide an airflow meter which is capable of reducing an influence of fluctuation of airflow.
According to a first aspect of the present invention, a sensing passage restricts a cross-sectional area of a bypass passage along a longitudinal direction of the bypass passage at least before a sensor element. The sensing passage is restricted in at least width in a lateral direction with respect to the sensor element. It is possible to define a restrictor in the bypass passage. Such a restrictor can be obtained by restricting the width in the lateral direction. Therefore, it is possible to reduce fluctuation and pulsation in the lateral direction. On the other hand, the arrangement enables relatively wider perpendicular width of the sensing passage on the sensor element. Therefore it is possible to protect the sensor element from damage caused by foreign particles or dust.
In addition, the sensing passage may be restricted in both width in the lateral direction and width in a perpendicular direction perpendicular to the sensor element. This arrangement is also effective to reduce fluctuation and pulsation.
In addition, the sensing passage may be defined as follows, 1.0xe2x89xa6L1/H, 1.0xe2x89xa6L2/H, and 2.5xe2x89xa6W/Hxe2x89xa620.0, wherein H is a maximum height of the surface, L1 is a length in the longitudinal direction between a beginning point of restriction and the maximum height, L2 is a length in the longitudinal direction between the maximum height and an ending point of restriction, and W is a width of the bypass passage. This arrangement is capable of providing an appropriate sensing characteristic, and protecting the sensor element from damage caused by foreign particle or dust.